This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The dlopen() function shall make the symbols (function identifiers
and data object identifiers) in the executable object file specified
by file available to the calling program.
The class of executable object files eligible for this operation and
the manner of their construction are implementation-defined, though
typically such files are shared libraries or programs.
Implementations may permit the construction of embedded dependencies
in executable object files. In such cases, a dlopen() operation shall
load those dependencies in addition to the executable object file
specified by file. Implementations may also impose specific
constraints on the construction of programs that can employ dlopen()
and its related services.
A successful dlopen() shall return a symbol table handle which the
caller may use on subsequent calls to dlsym() and dlclose().
The value of this symbol table handle should not be interpreted in
any way by the caller.
The file argument is used to construct a pathname to the executable
object file. If file contains a <slash> character, the file argument
is used as the pathname for the file. Otherwise, file is used in an
implementation-defined manner to yield a pathname.
If file is a null pointer, dlopen() shall return a global symbol
table handle for the currently running process image. This symbol
table handle shall provide access to the symbols from an ordered set
of executable object files consisting of the original program image
file, any executable object files loaded at program start-up as
specified by that process file (for example, shared libraries), and
the set of executable object files loaded using dlopen() operations
with the RTLD_GLOBAL flag. As the latter set of executable object
files can change during execution, the set of symbols made available
by this symbol table handle can also change dynamically.
Only a single copy of an executable object file shall be brought into
the address space, even if dlopen() is invoked multiple times in
reference to the executable object file, and even if different
pathnames are used to reference the executable object file.
The mode parameter describes how dlopen() shall operate upon file
with respect to the processing of relocations and the scope of
visibility of the symbols provided within file. When an executable
object file is brought into the address space of a process, it may
contain references to symbols whose addresses are not known until the
executable object file is loaded.
These references shall be relocated before the symbols can be
accessed. The mode parameter governs when these relocations take
place and may have the following values:
RTLD_LAZY Relocations shall be performed at an implementation-
defined time, ranging from the time of the dlopen() call
until the first reference to a given symbol occurs.
Specifying RTLD_LAZY should improve performance on
implementations supporting dynamic symbol binding since a
process might not reference all of the symbols in an
executable object file. And, for systems supporting
dynamic symbol resolution for normal process execution,
this behavior mimics the normal handling of process
RTLD_NOW All necessary relocations shall be performed when the
executable object file is first loaded. This may waste
some processing if relocations are performed for symbols
that are never referenced. This behavior may be useful
for applications that need to know that all symbols
referenced during execution will be available before
Any executable object file loaded by dlopen() that requires
relocations against global symbols can reference the symbols in the
original process image file, any executable object files loaded at
program start-up, from the initial process image itself, from any
other executable object file included in the same dlopen()
invocation, and any executable object files that were loaded in any
dlopen() invocation and which specified the RTLD_GLOBAL flag. To
determine the scope of visibility for the symbols loaded with a
dlopen() invocation, the mode parameter should be a bitwise-inclusive
OR with one of the following values:
RTLD_GLOBAL The executable object file's symbols shall be made
available for relocation processing of any other
executable object file. In addition, symbol lookup using
dlopen(NULL,mode) and an associated dlsym() allows
executable object files loaded with this mode to be
RTLD_LOCAL The executable object file's symbols shall not be made
available for relocation processing of any other
executable object file.
If neither RTLD_GLOBAL nor RTLD_LOCAL is specified, the default
behavior is unspecified.
If an executable object file is specified in multiple dlopen()
invocations, mode is interpreted at each invocation.
If RTLD_NOW has been specified, all relocations shall have been
completed rendering further RTLD_NOW operations redundant and any
further RTLD_LAZY operations irrelevant.
If RTLD_GLOBAL has been specified, the executable object file shall
maintain the RTLD_GLOBAL status regardless of any previous or future
specification of RTLD_LOCAL, as long as the executable object file
remains in the address space (see dlclose(3p)).
Symbols introduced into the process image through calls to dlopen()
may be used in relocation activities. Symbols so introduced may
duplicate symbols already defined by the program or previous dlopen()
operations. To resolve the ambiguities such a situation might
present, the resolution of a symbol reference to symbol definition is
based on a symbol resolution order. Two such resolution orders are
defined: load order and dependency order. Load order establishes an
ordering among symbol definitions, such that the first definition
loaded (including definitions from the process image file and any
dependent executable object files loaded with it) has priority over
executable object files added later (by dlopen()). Load ordering is
used in relocation processing. Dependency ordering uses a breadth-
first order starting with a given executable object file, then all of
its dependencies, then any dependents of those, iterating until all
dependencies are satisfied. With the exception of the global symbol
table handle obtained via a dlopen() operation with a null pointer as
the file argument, dependency ordering is used by the dlsym()
function. Load ordering is used in dlsym() operations upon the global
symbol table handle.
When an executable object file is first made accessible via dlopen(),
it and its dependent executable object files are added in dependency
order. Once all the executable object files are added, relocations
are performed using load order. Note that if an executable object
file or its dependencies had been previously loaded, the load and
dependency orders may yield different resolutions.
The symbols introduced by dlopen() operations and available through
dlsym() are at a minimum those which are exported as identifiers of
global scope by the executable object file. Typically, such
identifiers shall be those that were specified in (for example) C
source code as having extern linkage. The precise manner in which an
implementation constructs the set of exported symbols for an
executable object file is implementation-defined.
Upon successful completion, dlopen() shall return a symbol table
handle. If file cannot be found, cannot be opened for reading, is not
of an appropriate executable object file format for processing by
dlopen(), or if an error occurs during the process of loading file or
relocating its symbolic references, dlopen() shall return a null
pointer. More detailed diagnostic information shall be available
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 DLOPEN(3P)